diff --git a/j1939/j1939_21.py b/j1939/j1939_21.py
index 2ce1562..0a54f6a 100644
--- a/j1939/j1939_21.py
+++ b/j1939/j1939_21.py
@@ -305,6 +305,7 @@ def _process_tp_cm(self, mid, dest_address, data, timestamp):
                     'deadline': time.time() + self.Timeout.T2,
                     'src_address' : src_address,
                     'dest_address' : dest_address,
+                    'next_seq_num' : 1,
                 }
 
             self.__send_tp_cts(dest_address, src_address, self._rcv_buffer[buffer_hash]['num_packages_max_rec'], 1, pgn)
@@ -367,6 +368,7 @@ def _process_tp_cm(self, mid, dest_address, data, timestamp):
                     "deadline": time.time() + self.Timeout.T1,
                     'src_address' : src_address,
                     'dest_address' : dest_address,
+                    'next_seq_num' : 1,
                 }
             self.__job_thread_wakeup()
         elif control_byte == self.ConnectionMode.ABORT:
@@ -390,9 +392,19 @@ def _process_tp_dt(self, mid, dest_address, data, timestamp):
             # TODO: LOG/TRACE/EXCEPTION?
             return
 
+        if self._rcv_buffer[buffer_hash]['next_seq_num'] != sequence_number:
+            logger.critical(f"packet error. out-of-sequence pgn: {self._rcv_buffer[buffer_hash]['pgn']} "
+                            f"required: {self._rcv_buffer[buffer_hash]['next_seq_num']}  "
+                            f"received: {sequence_number}")
+            del self._rcv_buffer[buffer_hash]
+            self.__job_thread_wakeup()
+            return
+
         # get data
         self._rcv_buffer[buffer_hash]['data'].extend(data[1:])
 
+        self._rcv_buffer[buffer_hash]['next_seq_num'] = sequence_number + 1
+
         # message is complete with sending an acknowledge
         if len(self._rcv_buffer[buffer_hash]['data']) >= self._rcv_buffer[buffer_hash]['message_size']:
             logger.info("finished RCV of PGN {} with size {}".format(self._rcv_buffer[buffer_hash]['pgn'], self._rcv_buffer[buffer_hash]['message_size']))